Responses to single pulse electrical stimulation identify epileptogenesis in the human brain in vivo
Valentín A, Anderson M, Alarcón G, García Seoane JJ, Selway R, Binnie CD and Polkey CE;
Commented by , 25 Sep 2002
Background
The pathophysiology of neuronal activation in epileptic tissue is thought to rely on an imbalance between excitatory and inhibitory mechanisms. Surgical treatment has successfully been used in partial seizures, although better interictal measurements are needed in order to localize potential hyperexcitable and epileptogenic cortex in the human brain.
Aim
To investigate in vivo cortical excitability in the human brain.
Methods
The authors studied 45 consecutive patients with medically refractory epilepsy who were being evaluated for resective surgery. Subdural or intracerebral depth electrodes were implanted, topographically guided by previous findings from clinical history, neuroimaging, neuropsychology and scalp EEG recordings.
Between 1 and 20 seizures (median = 5) were recorded in each patient during the period of telemetry (3-22 days, median = 9). Ictal onset was identified independently by two electroencephalographers. The site of seizure onset was determined and classified as focal, regional and diffuse. Single pulse electrical stimulation was performed between adjacent electrodes using a constant current neurostimulator.
Results
Two main types of responses were seen, early responses with spikes and/or slow waves starting within 100 ms after stimulation (all patients), and delayed responses with spikes or sharp waves occurring between 100 ms and 1 s after stimulation (27 patients).
Early responses were seen in most regions and seem to be a normal response to single pulse stimulation. The distribution of delayed responses were significantly associated with the regions were seizure onset occurred.
Discussion
The delayed responses are considered to be pathophysiological in nature and presumably represent disinhibited cortical loops and thereby potential epileptogenic foci. The authors find that delayed responses identify the epileptogenic cortex as reliable as the study of seizure onset.
These observations may possibly lead to a new way of predicting the topography of seizure onset interictally. This would especially be of interest in patients who do not have seizures during video monitoring, or as guidance during implantation of intracranial electrodes in the process of identifying a focal seizure onset.
During the actual surgical procedure, single pulse stimulation can be performed in patients who have not undergone invasive recording to confirm localization and guide resection or give rise to further recording.